It is well known in systems for regulating temperature in an air conditioned space to use outside air to assist in maintaining the desired air temperature when the outdoor air conditions are suitable for that purpose. Suitability of the outside air for cooling depends on its enthalpy which is a function of both its temperature and humidity. Outside air is typically brought into the air conditioned space through economizer equipment.
Conventional economizer equipment provides significant economic benefits in many space cooling applications. However, it lacks flexibility in some situations. For example, it does not accommodate purposes other than cooling for admitting or limiting the admission of outdoor air. Under some circumstances, such as if indoor air quality falls below a quality standard, it may be desirable to admit more outdoor air than optimum for cooling purposes. Conversely, under other circumstances, such as if the outdoor air quality is poor or if the temperature of air entering the mechanical cooling equipment is too low, it may be desirable to limit the admission of outdoor air even though outdoor air could assist the cooling process.
To insure healthy and comfortable working or living conditions in large buildings, codes generally require that a minimum amount of fresh air be supplied to any such building during its occupancy. However, due to cost reduction measures, many building manufacturers have chosen to seal the building shell to reduce exfiltration of indoor air and resultant heat loss. These construction methods can result in buildings which are not adequately ventilated, with attendant degradation of indoor air quality. Although air quality may be deficient in a variety off respects, poor indoor air quality is often accompanied by an elevated carbon dioxide level.
Particularly, in northern climates, mechanical cooling may encounter problems if the temperature of admitted outdoor air approaches or drops below freezing. In such installations, many mechanical cooling systems employ preheat coils upstream from the cooling coils to provide for maintaining the air supplied to the cooling coils at a temperature above that which could cause the coils to freeze. Obviously, preheating air supplied to the cooling coils in preparation for cooling the air has an adverse impact on energy efficiency.
The applicant has devised several improvements for economizer controls which offer substantially increased operational flexibility. Among other things, the applicant's system provides for coordinated regulation of temperature and non-temperature air parameters to ensure optimum indoor air conditions, while protecting sensitive and costly equipment, and achieving operating efficiency.